The present invention relates to a method of forming a solid of a ferroelectric or a high dielectric material represented by a ferroelectric thin film employed in a semiconductor device, such as a ferroelectric memory. The present invention also relates to a method of manufacturing a semiconductor device, such as a ferroelectric memory.
A ferroelectric memory is a non-volatile storage device employing a ferroelectric film as a charge holding capacitor, and excels in high speed, low power consumption, high integration and rewriting resistance characteristics. A polarization induced by applying an electric field to the ferroelectric film remains after the electric field is lost. This makes it possible to achieve a non-volatile storage function.
FIG. 16 is a cross section showing a cell structure of the ferroelectric memory. A semiconductor substrate 1 is provided with an element forming region on its principal surface, which is isolated by a field oxide film 2, and impurity diffusion layers 3 and 4 spaced apart from each other are formed therein while a gate electrode 6 is formed on the principal surface of the semiconductor substrate 1 in the spacing between the impurity diffusion layers 3 and 4 by interposing a gate insulation film 5. In this manner, a transistor TR is formed.
The gate electrode 6 is coated by a first interlayer insulation film 7, over which a capacitor structure C such that sandwiches a ferroelectric film 10 between a lower electrode 11 and an upper electrode 12 is provided.
The upper electrode 12 is coated by a second interlayer insulation film 8. A first aluminum wire 9 formed on the second interlayer insulation film 8 is brought into contact with the upper electrode 12 and impurity diffusion layer 4 through contact holes 14 and 15, respectively, thereby electrically connecting the upper electrode 12 and impurity diffusion layer 4.
In the ferroelectric memory of this cell structure, the impurity diffusion layer 3 forms a bit line, and the gate electrode 6 and lower electrode 11 form a word line and a plate line, respectively. Hence, by applying an adequate writing voltage across the bit line (impurity diffusion layer 3) and plate line (lower electrode 11) while applying a selective voltage to the word line (gate electrode 6) so as to allow conduction in the transistor TR, an electric field can be applied to the ferroelectric film 10. Consequently, a polarization can be induced in the ferroelectric film 10 in an amount corresponding to the direction and intensity of the applied electric field.
At the time of reading, an adequate selective voltage is applied to the word line (gate electrode 6) so as to allow conduction in the transistor TR, while applying an adequate reading voltage to the plate line (lower electrode 11). A potential appearing in the bit line (impurity diffusion layer 3) at this point takes either one of two different potentials depending on the direction of the polarization in the ferroelectric film 10. Based on the foregoing, it is possible to check whether the cell is in the xe2x80x9c1xe2x80x9d state or xe2x80x9c0xe2x80x9d state.
In case that a multi-layer wiring is necessary as is shown in FIG. 16, the first aluminum wire 9 is further coated by a third interlayer insulation film 16. Then, a second aluminum wire 17 is additionally formed on the third interlayer insulation film 16, and connected to the first aluminum wire 9 through a contact hole 18. Further, the second aluminum wire 17 is coated by a protection film 19.
Generally, complex oxide ferroelectrics represented by those based on PZT (Pb(Zr,Ti)O3) and those based on SBT (SrBi2Ta2O9) are generally used as the materials of the ferroelectric film. Thin films of these ferroelectrics are formed by, for example, the sol-gel method. The sol-gel method is defined as a method of obtaining a necessary film by coating a liquid (sol) of a raw material over a substrate followed by calcining by means of annealing. In the sol-gel method of PZT, for example, a solution of organic compounds containing metal elements, that is, Pb(CH3COO)2.3H2O, Zr(nxe2x80x94OC4H9)4, and Ti(ixe2x80x94OC3H7)4, in a solvent of 2-methoxy ethanol is used as a starting material. The organic compound solution is spin-coated over the substrate and dried at 150xc2x0 C. to 180xc2x0 C., after which precalcining is carried out for 30 minutes at 400xc2x0 C. in a drying air atmosphere. This process is repeated until a predetermined film thickness is achieved, and finally, annealing at 600xc2x0 C. to 700xc2x0 C. is carried out to crystallize the film entirely.
However, crystallization at such high temperatures deteriorates element characteristics of the transistor TR formed beforehand, besides, mutual-diffusion of film materials at the interfaces between the ferroelectric film 10 and the upper and lower electrodes 11 and 12 causes characteristics deterioration of the ferroelectric film 10 itself. For this reason, a ferroelectric memory with satisfactory characteristics is not necessarily achieved.
The reason why the crystallization at such high temperatures is necessary is because the pre-crystallized film includes residual organic substances. Although the precalcining at a temperature of approximately 400xc2x0 C. can remove the organic substances to some extent, annealing at a temperature exceeding 700xc2x0 C. is necessary in order to remove the organic substances from the film in a satisfactory manner. Such a high temperature, however, causes crystallization of the film materials, thereby causing not only a loss of the purpose of the precalcining, but also more serious damages to the transistor TR formed on the semiconductor substrate 1.
Therefore, there is no conventional method of forming a ferroelectric film that has been crystallized satisfactorily by annealing at low temperatures, which makes it impossible to provide a ferroelectric memory with satisfactory characteristics.
On the other hand, because the complex oxide ferroelectrics represented by those based on PZT (Pb(Zr,Ti)O3) and those based on SBT (SrBi2Ta2O9) are oxides, they are vulnerable to a reduction atmosphere. Hence, if they undergo an interlayer insulation film forming process that uses SiH4, a H2 sintering process aiming at stabilizing the Pxe2x80x94N junction or improving ohmic characteristics at a contact, etc., the capacitor characteristics may deteriorate.
To be more specific, in case of forming the cell structure shown in FIG. 16, because the second and third interlayer insulation films 8 and 16 and the protection film 19 are formed after the ferroelectric film 10 is formed, it is unavoidable for the ferroelectric film 10 to be exposed in a reduction atmosphere.
In addition, because the ferroelectrics also have the piezoelectric characteristics, they are quite sensitive to stress applied from the interlayer insulation film or protection film, thereby possibly causing biased characteristics.
Hence, the ferroelectric film 10 is under the stress applied from the upper electrode 12, second and third interlayer insulation films a and 16, first and second aluminum wires 9 and 17, and protection layer 19, all of which being formed in the steps carried out after the ferroelectric film 10 is formed, and for this reason, capacitor characteristics as designed may not be necessarily achieved.
Further, in the steps carried out after the ferroelectric film 10 is formed, etching is indispensable to pattern the upper electrode 12, first and second aluminum wires 9 and 17, etc. However, this etching causes damages to the ferroelectric film 10, which is one of the factors that deteriorate the capacitor characteristics of the ferroelectric film 10.
It has been known that the characteristics deterioration of the ferroelectric film 10 as discussed above is restorable by annealing at 550xc2x0 C. to 600xc2x0 C. in an oxygen atmosphere. However, annealing at such high temperatures not only causes characteristics deterioration of the transistor TR, but also melts the aluminum wires 9 and 17. Therefore, it is impossible to apply annealing at or above 400xc2x0 C. once the aluminum wire 9 is formed.
As has been discussed, as to the ferroelectric film 10 employed as a capacitor film of the ferroelectric memory, there is virtually no means to restore the characteristic deterioration, and for this reason, a ferroelectric memory employing a ferroelectric film with satisfactory characteristics has not been necessarily achieved.
A first object of the present invention is to provide a method of forming a solid of a ferroelectric or a high dielectric material out of organic compound materials containing metal elements, by which a satisfactory solid of a ferroelectric or a high dielectric material can be formed by annealing at a relatively low temperature.
Also, a second object of the present invention is to provide a method of manufacturing a semiconductor device, by which a satisfactory functional thin film can be formed on a semiconductor substrate by annealing at a relatively low temperature, thereby making it possible to achieve a semiconductor device with satisfactory characteristics.
A third object of the present invention is to provide a method of manufacturing a semiconductor device, by which characteristics deterioration of a functional thin film can be restored in a satisfactory manner, thereby making it possible to manufacture a semiconductor device with excellent characteristics.
Also, a more concrete object of the present invention is to provide a method of manufacturing a semiconductor device, by which characteristics deterioration of a functional thin film can be restored by annealing at a relatively low temperature, thereby making it possible to restore characteristics deterioration of the functional thin film in a satisfactory manner.
A further concrete object of the present invention is to provide a method of manufacturing a semiconductor device capable of restoring characteristics deterioration of a ferroelectric film serving as a functional thin film.
The present invention provides a method of forming a solid of a ferroelectric or a high dielectric material by calcining organic compounds containing metal elements, comprising: a step of forming a film by coating a solution of organic compound material containing a metal element over a substrate; an organic substance removing step of removing organic substances from said film by applying organic substance removing treatment that uses means other than heat to organic compound materials containing metal elements, thereby obtaining inorganic compound material; and a crystallizing step of calcining to crystallize the inorganic compound material obtained in the organic substance removing step, thereby obtaining a solid of a ferroelectric or a high dielectric material.
According to this method, by using means other than heat, the organic substances that will become crystallization-inhibiting factors can be fully removed. Moreover, because calcining for crystallization is carried out after the organic substances have been removed, it is possible to crystallize the inorganic compound material by calcining at a relatively low temperature, thereby obtaining a solid of a ferroelectric or a high dielectric material. Consequently, mutual-diffusion of material of the solid of a ferroelectric or a high dielectric material and any other solid adjacent to the same can be prevented, and if there is a solid portion to be formed as an integral part of the solid of a ferroelectric or a high dielectric material, a thermal effect on that solid portion can be suppressed.
In other words, it is preferable that the crystallizing step is carried out at a temperature lower than a temperature, at or above which material of the solid of a ferroelectric or a high dielectric material and any other solid adjacent to the same start to diffuse each other. Likewise, it is preferable that the crystallizing step is carried out at or below a certain temperature predetermined so as to prevent a thermal effect to a solid portion to be formed as an integral part of the solid of a ferroelectric or a high dielectric material.
Said step of forming a film may include a step of performing precalcining after said solution coated on said substrate is dried.
It is preferable that the organic substance removing step includes a depressurizing step of placing the organic compound material in a low-pressure atmosphere. Accordingly, because the organic compound material is placed in a low-pressure atmosphere, evaporation of the organic substances is accelerated. As a result, the organic substances can be removed efficiently.
Also, it is preferable that heat treatment is carried out in parallel with the depressurizing step at a temperature such that does not cause crystallization. Also, it is preferable that the heat treatment in this case is carried out at a temperature lower than a temperature, at or above which materials of the organic compound material and any other solid adjacent to the same start to diffuse with each other. Likewise, it is preferable to carry out the heating treatment at a certain temperature predetermined so as to prevent a thermal effect to a solid portion to be treated together with the organic compound materials.
It is preferable that the crystallizing step is carried out after the depressurizing step. In this case, because the crystallizing step is carried out after the organic substances in the organic compound material are removed in a reliable manner by the depressurizing step, crystallization of the inorganic compound material can proceed in a satisfactory manner.
For example, the organic substance removing step including the depressurizing step, and the crystallizing step can be carried out by different treatment systems. More specifically, for example, the organic substance removing step may be carried out by a lamp heating device having a low-pressure treatment chamber, and the crystallizing step may be carried out by a heating furnace.
The depressurizing step and crystallizing step may be carried out almost simultaneously by calcining the organic compound materials in a low-pressure atmosphere.
In other words, the organic substance removing treatment and crystallization are carried out by reducing an internal pressure of the chamber of the treatment system and heating the organic compound material within the chamber. In this case, the crystallization is inhibited while residual organic substances are present in the material, and crystallization starts after the organic substances are removed and inorganic compound material is obtained. Therefore, the inorganic compound solid can be obtained by calcining at a relatively low temperature.
The advantages of this method are that two steps can be carried out successively by a single treatment system, which makes it possible to simplify the process sequence, and that the cost can be saved.
It is preferable that the organic substance removing step includes a step of giving energy other than heat to the organic compound material.
By giving energy other than heat to the organic compound material, it is possible to remove the organic substances in the organic compound material. Hence, the organic substances can be removed in a satisfactory manner without heating the organic compound material at high temperatures, and therefore, it is possible to prevent mutual-diffusion of materials of the solid of a ferroelectric or a high dielectric material and any other solid adjacent to the same, and a thermal effect to a solid portion to be formed as an integral part of the solid of a ferroelectric or a high dielectric material.
It should be appreciated, however, that thermal energy can be given to the organic compound material together with energy other than heat to the extent that the mutual-diffusion of material and thermal effect to the solid portion are the least.
The step of giving energy other than heat may include an electromagnetic wave supplying step of supplying electromagnetic waves to the organic compound material.
Examples of the electromagnetic waves include UV rays, microwaves, etc.
Besides the electromagnetic waves, the organic substance removing treatment can be carried out by giving energy to the organic compound material by activated particles, such as plasma.
Also, the step of giving energy other than heat may include a step of treating the organic compound material with activated oxygen particles.
Examples of the activated oxygen particles include ozone (O3), oxygen radicals, oxygen ions (O++, O+), etc.
Bringing the organic compound material into contact with the activated oxygen particles makes it possible to give energy to the organic substances in the material, thereby achieving the organic substance removing treatment.
It is more effective when this treatment is carried out together with heat treatment or annealing to the organic compound materials. It is preferable that the annealing in this case is carried out at a temperature such that does not cause crystallization of the organic compound materials. Also, it is preferable that the annealing is carried out at a temperature such that does not cause mutual-diffusion of materials between the inorganic compound solid and any other solid adjacent to the same. Further, in case that the solid of a ferroelectric or a high dielectric material is formed together with another solid portion into one body, it is preferable that the annealing is carried out at a temperature such that a thermal effect given to the solid portion is the least.
Examples of the ferroelectric include complex oxides represented by PZT (Pb(Zr,Ti)O3) and SBT (SrBi2Ta2O9), etc. A method of manufacturing a semiconductor device of the present invention is characterized by comprising a step of forming, on a semiconductor substrate, a functional thin film made of the solid of a ferroelectric or a high dielectric material formed by the foregoing methods. The functional thin film may be a capacitor film, and the capacitor film may be made of a ferroelectric.
According to this invention, because the functional thin film can be formed by a process at a relatively low temperature, mutual-diffusion of materials between the films and a thermal effect to a functional element formed on the semiconductor substrate can be prevented, thereby making it possible to achieve a semiconductor device with satisfactory characteristics.
In other words, it is preferable that the functional thin film forming step is carried out at a temperature such that does not cause diffusion of materials at the interface of the films, and in case that the functional element is formed on the semiconductor substrate, at a temperature such that does not deteriorate the characteristics of the functional element.
It is preferable that this method includes an element forming step of forming the functional element on the semiconductor substrate before the functional thin film forming step.
In this invention, because the functional thin film can be formed by annealing at a low temperature, the characteristics of the functional element formed before the functional thin film will not be deteriorated.
It is preferable that the crystallizing step is carried out at or below a certain temperature predetermined so as not to deteriorate the characteristics of the functional element.
Consequently, deterioration of the characteristics of the functional element can be prevented in a reliable manner, thereby making it possible to achieve a semiconductor device with satisfactory characteristics.
Examples of the functional element include a transistor, such as a field effect transistor, a capacitor, a resistor, etc.
It is preferable that the crystallizing step is carried out at a predetermined temperature lower than a temperature, at or above which mutual-diffusion of materials occurs between the functional thin film and a solid adjacent to the same.
Consequently, mutual-diffusion of materials between the functional thin film and a solid adjacent to the same (other thin films or the like) can be prevented in a reliable manner, thereby making it possible to achieve a semiconductor device with satisfactory characteristics.
In case that the functional thin film is a ferroelectric thin film, it is possible to fabricate a ferroelectric storage device employing the ferroelectric thin film as a charge holding film.
According to this invention, because a ferroelectric thin film satisfactorily crystallized by annealing at a relatively low temperature can be employed as the charge holding film, a satisfactory ferroelectric storage device can be achieved. In particular, in case that a writable non-volatile storage device is realized by exploiting the polarization holding characteristics of the ferroelectrics, significant improvements are achieved in terms of inversion polarization characteristics, writable number of times, low voltage driving, etc.
Another aspect of the present invention relates to a method of manufacturing a semiconductor device comprising: a step of forming a functional thin film on a semiconductor substrate; and
a restoring step of restoring characteristics deterioration of the functional thin film caused by influences during steps carried out after the functional thin film is formed, which method being characterized in that the restoring step includes: a treatment step of giving energy other than heat to the functional thin film; and a heat treatment or an annealing step of giving thermal energy to the functional thin film.
The treatment step of giving energy other than heat to the functional thin film and annealing step in the restoring step may be carried out in such a manner that the former can be carried out before the latter or vice versa. However, it is preferable to carry out the both steps simultaneously.
According to the present invention, by using energy other than heat and thermal energy together, the characteristics deterioration of the functional thin film is restored. For this reason, only a small amount of thermal energy has to be given to the semiconductor substrate in the restoring step. Consequently, a thermal effect to the portions other than the functional thin film is lessened. On the other hand, the functional thin film can be supplied with sufficient energy as both the energy other than heat and thermal energy are given. As a result, the functional thin film having undergone the restoring step can have satisfactory characteristics. In other words, characteristics deterioration of the functional thin film can be recovered in a satisfactory manner by annealing at a relatively low temperature.
The functional thin film may be a complex oxide thin film. Examples of complex oxides forming the complex oxide thin film include PZT (Pb(Zr,Ti)O3) and SBT (SrBi2Ta2O9).
The complex oxide film is caused deterioration in characteristics (in particular, capacitor characteristics) when exposed in a reduction atmosphere during an insulation film forming process or a H2 sintering step. Hence, the function restoration by the restoring step discussed above is needed frequently.
The restoring step may further include an oxygen introducing step of introducing an oxidation gas to a surface of the semiconductor substrate having formed thereon the functional thin film.
The oxidation gas is a gas containing oxygen, examples of which including oxygen gas (O2), ozone (O3), Nox, etc.
It is preferable that the oxygen introducing step is carried out simultaneously with non-annealing step (treatment step of giving energy other than heat to the functional thin film), and/or annealing step.
According to this invention, the semiconductor substrate can be placed in an oxygen gas atmosphere during the restoring step of restoring the characteristics of the functional thin film. This promotes oxidation of the functional thin film with deteriorated characteristics due to exposure in a reduction atmosphere, thereby making it possible to restore the characteristics in a satisfactory manner. Hence, the oxygen introducing step is particularly effective when the functional thin film is made of the complex oxides.
The treatment step of giving energy other than heat to the functional thin film may include an oxygen activated particle treatment step of placing the semiconductor substrate having formed thereon the functional thin film in an oxygen activated particle atmosphere.
Examples of the oxygen activated particles include ozone, oxygen radicals, plasma, etc.
By placing the semiconductor substrate in the oxygen activated particle atmosphere, energy can be given to the functional thin film, thereby making it possible to restore the characteristics deterioration of the functional thin film. In particular, when the functional thin film is made of complex oxides, the damages can be restored as the oxygen in the atmosphere is activated.
In this case, it is more preferable that an oxidation gas, such as an oxygen gas, is introduced in the vicinity of the semiconductor substrate, because by so doing, the characteristics of the functional thin film can be restored more effectively.
The treatment step of giving energy other than heat to the functional thin film may include an electromagnetic wave supplying step of supplying an electromagnetic wave to the functional thin film. By supplying the electromagnetic waves to the functional thin film, it is possible to restore the characteristics of the functional thin film with a supply of energy other than heat.
Examples of the electromagnetic waves include UV rays, microwaves, etc.
The functional thin film may be a ferroelectric film. In this case, even when the capacitor characteristics and polarization characteristics of the ferroelectric film are deteriorated during various steps carried out after the ferroelectric film is formed, such characteristics deterioration can be recovered in a satisfactory manner.
The semiconductor device may be a ferroelectric storage device employing the ferroelectric film as a charge holding film.
In this case, because the capacitor characteristics and polarization characteristics of the ferroelectric film can be restored in a satisfactory manner by the restoring step, a storage device (memory) with excellent characteristics can be achieved.
The above method may further include a wiring forming step of forming a wiring on the semiconductor substrate before the restoring step.
In the restoring step of restoring the function of the functional thin film with deteriorated characteristics, both thermal energy and energy other than heat are used, and therefore, the characteristics can be recovered at a relatively low temperature. As a result, the characteristics of the functional thin film can be restored without giving damages to the wiring, thereby making it possible to achieve a semiconductor device with satisfactory characteristics.
In other words, by carrying out the annealing step such that a temperature of the semiconductor substrate does not exceed a certain temperature predetermined so as not to deteriorate the wiring, no damage is given to the wiring.
For example, in case that the wire is made of aluminum, it is preferable that the certain temperature is approximately 400xc2x0 C. or below.
The method may further include an element forming step of forming a functional element on the semiconductor substrate before the restoring step.
Examples of the functional element include a transistor, such as a field effect transistor, a capacitor, a resistor, etc.
According to this invention, because the characteristics of the functional thin film can be restored at a relatively low temperature, no damage is given to the functional element that is formed before the restoring step. As a result, a semiconductor device with satisfactory characteristics can be achieved.
In other words, by carrying out the annealing step such that a temperature of the semiconductor substrate does not exceed a certain temperature predetermined so as not to deteriorate the functional element, the characteristics of the functional element will never be deteriorated.
For example, in order to protect the functional element, such as a transistor, formed on the semiconductor substrate, it is preferable that the above certain temperature is approximately 400xc2x0 C. or below.